Antenna drive system



March 12, 1968 M, SUUTEANQ ET AL 7 3,372,603

ANTENNA DRIVE SYSTEM Filed Aug. 2, 1965 2 Sheets-Sheet 1.

IFIIEI1 so INVENTORS MENAHEM SULITEANU FREDERICK M. FONDA,JR.

March 12, 1968 M. SULITEANU ET AL 3,372,603

ANTENNA DRIVE SYSTEM Filed Aug. 2, 1965 2 Sheets-Sheet 2 MOTOR 6 TACH COMPENSATION ,V LOAD NETWORK I l 53 I IO 50 I I DRIVE ETwoRK 29 J I 22 SUMMING SUMMING JUNCTION JUNCTION POSITION ERRoR SIGNAL SUMMING 28 DRIVE JUNCTION TRAIN DRIVE NETWORK I I I I I 54 COMPENSATION //57 NETWORK MOTOR I 55 COMPENSATION /58 TACH NETWORK LOAD 1 TACHOMETER INVENTORS 27 MENAHEM SULITEANU 2; 32 I j w ATTO RN EY FREDERICK M. FONDA,JR.

United States Patent ABSTRACT OF THE DISCLOSURE A cylindrical drum of a rotatable antenna is driven by engagement of the drum by a pair of opposed reversible endless belt drive assemblies which form part of a closed loop servo system. The periphery of the drum comprises a thin layer of elastic material which is bonded to a hard cylindrical surface and viscously damps resonance effects between driving and driven structures.

This invention relates to drive systems, and more particularly to an anti-resonant drive system for automatic tracking antennas and the like in which driving power in two directions is transmitted to the driven member.

Large antenna systems are becoming an increasingly important tool for tracking and communicating with space vehicles. The large movable antenna structures required for such applications must be controlled with a high degree of accuracy. One phenomenon which directly affects the positional accuracy and speed of response of large reflectors, for example, is the mechanical resonance of the moving structure. When such a structure is moved rapidly and alternately in opposite directions at a frequency corresponding to natural frequency of the structure, a small exciting force Will induce large vibrating inertial forces and thus cause severe stresses in the members. In order to insure that the resonant frequency of the moving structure and drive system is Well spaced from the required frequency response for the system, practice in the past has been to increase the stiffness of the structure. This means increased mass and Weight of antenna pedestals and azimuth ring gears as well as the addition of heavy counterweights. In addition to the obvious resultant disadvantages, i.e., increased cost, power consumption, and wear, only limited improvement in system performance can be achieved by increasing stiffness. For example, a normal safety factor for high performance antenna systems is that the ratios between load-gearing resonant frequency and position loop cutoff frequency extend from a minimum of 8:1 to :1 or more. It the position loop is viewed as a damped, two-pole, tow-pass filter, these ratios imply that the physical stiffness required in the gearing must exceed the required position loop stiffness by factors of 64:1 to 100:1 or more.

In accordance with this invention, a drive system with inherent damping characteristics is utilized. A closed-loop servo control system provides driving power to the driven member through a viscoelastic coupling. In one form of the invention, two motors which drive in opposite directions engage a compliant endless belt which in turn frictionally engages a substantial portion of the surface of an elastic sleeve on the periphery of the driven drum member. Thus viscoelastic transmission of drive energy in the closed-loop servo control system provides effective inherent damping in the drive system to facilitate compensation of system resonance. As a result of the viscous of the system and furthermore eliminates the resonant amice plifieation of energy at the natural frequency of the system.

A general object of the invention is the provision of an extremely precise controlled position drive system with inherent damping characteristics which substantially overcome the resonance frequency effects in the drive mechanism and driven member.

Another object is the provision of a closed-loop servo drive system with inherent damping characteristics which permit lighter, less massive driven structures to be rapidly and precisely positioned without deleterious structural resonance effects.

A further object is the provision of a self-damping torque coupling for positioning through closed-loop control techniques a structure having a stiffness which is substantially independent of the closed-loop stability requirements.

These and other objects of the invention will become apparent from the following description of a preferred embodiment thereof, reference being bad to the accompanying drawings in which:

FIGURE 1 is a front view of an antenna structure with azimuth and elevation drive systems embodying this invention;

FIGURE 2 is a transverse section taken on line 22 of FIGURE 1 showing a plan of the azimuth drive system;

FIGURE 3 is a transverse section taken on line 3-3 of FIGURE 2;

FIGURES 4 and 5 are sections similar to FIGURE 3 showing modified forms of viscoelastic drive coupling; and

FIGURE 6 is a block diagram of a closed-loop servo drive system with which this invention may be practiced.

Referring now to the drawings, an antenna assembly 10 is illustrated. in FIGURE 1 as an example of a structure to be positioned rapidly and accurately and which is dimensionally large. The antenna assembly 10 comprises a reflector unit 12, which may be 3 feet to 60 feet or more in diameter, supported for rotation about a transverse axis 14 by a semi-circularly-shaped upwardly extending yoke 15. The upper part 16a of a cradle 16 supports the yoke at its circular periphery for rotation about cross-axis 17 and the lower horizontal portion 16b of the cradle is in turn supported on base 18 for rotation about a vertical or azimuth axis 19.

This antenna pedestal is described in greater detail in the copending application of Menahem Suliteanu, Ser. No. 459,763, filed May 28, 1965, now Patent No. 3,313,502.

The lower horizontal portion 16b of the yoke has a coaxial collar 22, see FIGURE 2, defining the peripheral side surface of this part of the yoke. Collar 22 constitutes a cylindrical drum to which driving torque is applied for rotating the antenna in azimuth about axis 19. This torque is developed by two substantially identical drive assemblies 24 and 25 located adjacent to drum 22 at diametrically opposite positions. Radial forces applied to the drum by these drives are thus balanced. Each azimuth drive assembly comprises an endless, preferably fiat belt 26, arcuately spaced sheaves 27 and 28 which frictionally engage the belt, and drive motors 29 and 30 which have direct drive connections with sheaves 27 and 28, respectively. Motors 29 and 30 are mounted on the side of the base 18 by brackets 31. Belt 26 extends around sheaves 27 and 28 with the inside leg of the belt loop between the sheaves pressing against drum 22, and is driven in one direction by motor 29 and in the opposite direction by motor 30. Tension of belt 26 and thus the drive force it develops is adjusted by engagement of the outer leg of the belt by idler sheave 32 which is radially adjustably supported by brackets 33 on the side of the base between drive motors 29 and 30.

Since drive system 24 is substantially identical to drive system 25, like reference characters indicate like parts on the drawing.

In a preferred form of the invention, outer surface 22a of drum 22, see FIGURE 3, is covered by a smooth sleeve or layer 35 of a viscoelastic material permanently bonded thereto and adapted to be frictionally engaged by belt 26 for transmission ofdriving torque to the cradle. Belt 26 may be made of rubber, fiberglas or metal chain, is smooth, and is sufiiciently compliant to press tightly against the outer surface of the elastic layer for its full width and over a portion of its length slightly less than the arcuate distance between drive sheaves 27 and 28. The radial force with which belt 26 presses against the elastic layer covered drum is predetermined by adjustment of idler drum 32 so that the maximum value of friction drive force required to position the antenna is not exceeded, thus precluding slipping.

Belt 26 and layer 35 on the drum are elastic coupling elements. One or the other or both have viscoelastic properties exhibiting a substantial hysteresis loss characteristic. This hysteresis loss characteristic is the mechanism by which the damping properties of this coupling are realized since a relatively large proportion of energy used to induce strain in the material is dissipated in the material. The energy dissipated is a function of the rate of displacement of the material and therefore represents a true viscous damping process. The degree of damping is predetermined by proper selection of the thickness of the elastic layer in conjunction with both the arcuate length of engagement between belt and layer 35 and the belt tension (radial force) so that sufficient damping of the load and drive train resonance is achieved which then allows servo closed-loop compensation techniques to com pletely compensate the load drive train resonance. The

final drive system is thus designed so that the belt which transmits the drive force is viscoelastically coupled to the load.

By way of example, a material useful in forming the layer 35 in this coupling system is the elastomer urethane.

Modified forms of a viscoelastic coupling between the drive motors and drum 22 are shown in FIGURES 4 and 5. A belt 26' composed of elastic material such as rubber or fiberglas may directly engage a rigid smooth surface 22a of drum 22 as shown in FIGURE 4. The drum 22 as well as the horizontal portion 16b of the cradle may be made from lightweight structural material such as fiberglas. In the form of coupling shown in FIG- URE 4, the damping action at the coupling takes place substantially completely in the belt 26. Alternatively, a belt 26", see FIGURE 5, made of steel links or a combination of steel and rubber may be used for frictionally engaging a urethane layer 35 bonded to drum 22 so that substantially all of the damping action in the viscoelastic drive coupling takes place in layer 35.

Rotation of antenna in elevation about axis 14 is accomplished by application of torque to a drive ring 40 on the outboard side of journal 41 of the antenna by means of a belt 42 driven by two oppositely rotatable motors and associated sheaves, one set of which motors and sheaves is shown at 43 and 44, respectively, in FIGURE 1. A radially adjustable idler sheave, not shown, is also provided to adjust the tension of the belt. In this instance, belt 42 is wrapped around the greater portion of the periphery of ring 40 which is covered by an elastic layer similar to layer 35 on azimuth drum 22. The two drive motors rotate sequentially in opposite directions when selectively energized and cause the antenna 10 to rotate in the desired direction about axis 14. These elevation drive motors are, in accordance with this invention, thus viscoelastically coupled to the ring 40 in the same manner as azimuth drive motors 29 and 30 are coupled to the drum 22 as described above. Therefore the structural details of elevation drive viscoelastic coupling will not be repeated.

The difiiculty in avoiding destructive resonance forces in a dynamic system is markedly increased when the system includes positional feedback control as in automatic tracking antenna systems. Feedback control inherently has the essential ingredient for destructive system oscillation. As shown in FIGURE 6, a preferred servo loop control for the antenna azimuth drive system described above includes the motors 29 and (only one two-motor drive system is shown for simplicity), one motor applying torque in one direction and the other motor applying torque in the opposite direction to the load (antenna 10) through the drive train including drum 22. The motors are selectively energized by a position error signal applied to one of the motors through drive networks 50 and 51. Feedback signals are derived from tachometers 53 and 54 responsive to the rate of motor displacement and load tachometer 55 responsive to the rate of load displacement. Suitable compensation networks 56, 57 and 58 process the feedback signals into the drive networks through summing junctions.

Past techniques to accomplish the control of the loaddrive train resonance in closed-loop positioning systems have been limited in effectiveness, the one most frequently employed technique being attenuation of the position loop gain so that it is but a small fraction of unity at the load drive train resonance frequency. Therefore that frequency must be several octaves higher than the desired position closed-loop cutoff frequency. This technique places severe requirements on the weight and size of conventional systems because of the necessity of greater rigidity in the drive system.

The final torque multiplier in this drive loop is the friction drive system which consists of the relatively small diameter driving sheaves 27 and 28, the relatively large diameter cradle drum 22 having a bonded urethane layer for a friction surface, and the belt or chain which frictionally engages the sheaves and drum. The greater part of the belt between driving sheaves is in contact with the urethane layer. The viscoelastic properties of the urethane layer and the engagement of this layer by the compliant belt combine to produce all of the viscous coupling in this final torque multiplying system. Since all of the urethane layer in contact with the belt enters into the coupling, a flat belt is more effective than a toothed belt, such as a silent chain.

The end result of the closed-loop servo-system described above including compensation networks together with the viscoelastic coupling of the driving torque to the load is to provide control of the load train drive resonance to the extent that it is completely absent in the overall closed position loop response of the system regardless of the frequency selected for position loop gain crossover. In other words, the self-damping properties of the drive system make possible the positioning through closed-loop control techniques of a structure whose stiffness, and thus weight and mass, is not dictated by closed-loop stability requirements.

We claim:

1. A drive system for an automatic tracking antenna having a member supported for rotation about an axis and adapted to be driven in opposite directions comprismg a coaxial drum on said driven member, and

a closed loop servo control system for the transmission of driving energy to the driven member comprising first and second drive motors, first and second sheaves operatively connected to said motors, respectively, for rotation in -opposite directions when the respective motors are energized,

said sheaves being spaced closely to said drum at arcuately spaced positions thereabout, an endless belt frictionally engaging said sheaves and viscoelastically coupled to a substantial portion of said drum for transmitting torque from the motors to the driven member, network means connected to said motors for selectively energizing same in response to a position signal, and feedback means responsive to movement of said motors and said driven member for balancing the position signal and damping system structural resonance. 2. The drive system according to claim 1 in which said drum has an elastic sleeve bonded thereto over the entire drum periphery, said belt engaging said sleeve and transmitting torque to the driven member through the sleeve.

3. The drive system according to claim 2 in which said sleeve comprises urethane.

4. A drive system for an automatic tracking antenna having a member supported for rotation about an axis and adapted to be driven in opposite directions comprismg a coaxial drum on said driven member having an elastic Sleeve bonded thereto, and

a closed loop servo control system for the transmission of driving energy to the driven member comprismg first and second drive motors,

first and second sheaves operatively connected to said motors, respectively, for rotation in opposite directions when the respective motors are energized,

said sheaves being spaced closely to said drum at arcuately spaced positions thereabout, an endless belt frictionally engaging said sheaves and a substantial portion of said sleeve for viscoelastically transmitting torque from the motors to the driven member,

a third sheave supported intermediately of said first and second sheaves for engagement by said belt,

means for adjusting the position of said third sheave transversely of the direction of travel of said belt for changing the friction drive force applied to said sleeve,

drive networks connected to said motors for energizing same in response to a positioning signal, and

feedback means responsive to movement of the motors and driven member for balancing the positioning signal and damping system structural resonance.

5. The drive system according to claim 4 in which the surface of the belt which engages the sleeve is substantially flat.

6. The drive system according to claim 4 in which said belt engages less than degrees of sleeve-covered drum, and a second substantially identical drive system with a belt engaging said sleeve at a diametrically opposite location relative to the first described drive system.

7. A drive system for a structure supported for rotation about an axis comprising a coaxial drum on said structure,

a closed loop servo control system for rotating said structure in opposite directions comprising drive motor means supported adjacent to said drum,

a compliant belt driven by said motor means and viscoelastically coupled to said drum for transmitting torque to said structure,

network means connected to said motor means for energizing same to selectively drive the belt in one direction or a reverse direction in response to a position signal, and

feedback means responsive to movement of said drive means and said structure for balancing the position signal and compensating resonance elfects in the drive means and the structure.

8. The drive system according to claim 7 in which said drum has an elastic layer bonded to the periphery thereof, said layer exhibiting a substantial hysteresis loss characteristic whereby energy transmitted between the drive means and said structure is viscously damped.

9. The drive system according to claim 8 in which said layer is composed of urethane.

References Cited UNITED STATES PATENTS 2,451,899 10/1948 Yeomans et al. 74-221 2,509,054 5/1950 Davis 74-221 X 2,664,758 1/1954 Smits 74221 DONLEY I. STOCKING, Primary Examiner. L. H. GERIN, Assistant Examiner. 

